Technical Field
Embodiments of the subject matter disclosed herein generally relate to sensors for collecting seismic data and, more particularly, to mechanisms and techniques for providing such sensors to withstand shocks and other adverse conditions when deployed in the field.
Discussion of the Background
Seismic data acquisition and processing may be used to generate a profile (image) of the geophysical structure under the ground (either on land or seabed). While this profile does not provide an accurate location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of such reservoirs. Thus, providing a high-resolution image of the subsurface is important, for example, to those who need to determine where oil and gas reservoirs are located.
Traditionally, a seabed seismic survey is performed in the following way: Plural seismic sensors are electrically connected to each other and then stored on a vessel. The vessel travels to the area needing to be surveyed and deploys the seismic sensors to the seabed. However, during the deployment phase, the seismic sensors may be subjected to impacts (e.g., with the vessel, the crane handling the sensors, the water, the seabed, etc.). Because the seismic sensors are designed to detect small variations of a given parameter (e.g., displacement, speed, acceleration, pressure, etc.), the sensors' internal components are very sensitive to jarring or direct contact, and may be damaged by an unintentional shock.
After all the seismic sensors have been deployed, one or more seismic sources are brought and actuated to generate seismic waves, which propagate through the water and then through the seabed until they are reflected by various reflectors in the subsurface. The reflected waves propagate to the seismic sensors, where earth movement is recorded. However, if the seismic sensor is damaged because of a shock received during deployment, the recorded data is poor.
Two conventional seismic sensors, the geophone and the micro-electro-mechanical system (MEMS) accelerometer, and their limitations are now discussed. Geophone technology is based on electromagnetic induction. The geophone includes a magnet encapsulated by a moving electric coil. Movement of the electric coil in the magnetic field produced by the magnet induces a voltage in the coil. This voltage is a function of the velocity of the coil and, thus, the geophone's velocity. A disadvantage of the geophone is poor response at low frequencies and the presence of resonance inside the seismic band. Another disadvantage is that the magnet inside the geophone creates problems for other magnetic devices, such as electro-mechanical (EM) equipment and magnetic compasses embedded in other sensors to provide directional information.
The MEMS accelerometer has a microscopic finger that is sensitive to any shock or movement. Its relative capacitance to a reference is proportional to its movement. In other words, when the finger is subjected to acceleration, the capacitance between the reference and the finger will change due to a deformation of the finger. The change in capacitance is measured, and a signal is fed back to the finger via an electric field that forces the capacitive finger to equilibrium. The change in capacitance is modulated in a digital circuit and, thus, provides a signal output proportional to the acceleration. However, the conventional MEMS accelerometer has a relatively high noise level at low frequencies.
Other accelerometers have a piezoelectric component, and an amplifier converts the electric charge accumulated on the piezoelectric component to voltage. These piezoelectric accelerometers rely on the piezoelectric effect of quartz or ceramic crystals to generate an electrical output proportional to the applied acceleration. The piezoelectric effect produces an opposed accumulation of charged particles on the crystal. This charge is proportional to the applied force or stress. A force applied to a quartz crystal lattice structure alters the alignment of positive and negative charges, which results in an accumulation of these charges on opposed surfaces. These charges accumulate on an electrode that is connected to an for analysis.
A variety of mechanical configurations are available to perform the transduction principles of a piezoelectric accelerometer, and are defined by the way in which the inertial force of an accelerated mass acts upon the piezoelectric material.
For example, shear mode designs bond or “sandwich” the sensing crystals between a center post and a seismic mass. Under acceleration, the seismic mass causes a shear stress to be applied to the sensing crystal. Shear accelerometers are good in rejecting thermal transient and base bending effects. Also, shear geometry lends itself to small size, which minimizes mass loading effects on the test structure.
Another example of a piezoelectric accelerometer is a flexural mode design that utilizes beam-shaped sensing crystals, which are supported to create strain on the crystal when accelerated. The crystal may be bonded to a carrier beam that increases the amount of strain when accelerated. This design offers a low profile, light weight, good thermal stability, and a low price. Insensitivity to transverse motion is an inherent feature of this design. Generally, flexural beam designs are well-suited for low-frequency, low-gravity (low-g) level applications such as those which may be encountered during structural testing.
Compression mode accelerometers offer a simple structure, high rigidity, and high availability. Upright compression design offers high resonant frequencies, resulting in a broad, accurate frequency response range. This design is generally rugged and can withstand high-g shock levels. However, upright compression designs tend to be more sensitive to base bending (strain) and thermal transient effects. Inverted compression designs isolate the sensing crystals from the mounting base, reducing base bending effects and minimizing the effects of a thermally unstable test structure. Isolated compression designs reduce erroneous outputs due to base strain and thermal transients. These benefits are achieved by mechanically isolating the sensing crystals from the mounting base and utilizing a hollowed-out seismic mass that acts as a thermal insulation barrier. These mechanical enhancements allow stable performance at low frequencies, where thermal transient effects can create a signal “drift” with other compression designs.
Some of the materials used for piezoelectric accelerometers are now discussed. Quartz is a natural material commonly used in accelerometers and exhibits unmatched long-term stability. Lead zirconate titinate (PZT) is another common material used in accelerometers after they have been “polarized.” High shock levels or high-temperature installations may cause shifts in the output of PZT-based sensors. However, quartz exhibits superior temperature stability and has no aging effects and is, therefore, extremely stable over time. Quartz sensors offer high-voltage sensitivities and require voltage amplifiers to condition the signal. Voltage amplifiers, with large-valued resistors, are inherently noisier and limit the minimum measurable signal, but allow for very high levels of vibration to be monitored. PZT-based sensors provide a high charge output and a high capacitance. “Quieter” microelectronic charge amplifiers may be used, thus allowing the low-level vibrations to be measured.
When selecting accelerometers, the vibration specialist needs to consider three areas: amplitude range, frequency range, and environmental considerations. The amplitude range can be increased by either increasing the supply voltage level or decreasing the sensitivity of the accelerometer. The resolution (frequency range) of the sensor is determined by two factors: electrical noise of the internal amplifier and mechanical gain of the mass/piezoelectric system. The larger the seismic mass, the larger the output of the sensor prior to amplification. This high mechanical gain improves low-level measurements by producing substantial electrical signals without the use of amplifier gain. The frequency response of an accelerometer is described as the frequency range over which the sensor will provide a linear response. The upper end of the frequency response is governed by the mechanical stiffness and size of the seismic mass in the sensing element, while the low-frequency range is controlled by the amplifier roll-off and discharge time constant. A large seismic mass will also produce higher mechanical gain, thereby resulting in a lower noise accelerometer with greater sensitivity. A smaller seismic mass will produce lower signals but will result in a sensor with a higher resonant frequency.
However, a common problem of the above-noted accelerometers is that above a certain critical strain (the “elastic limit”), a material will plastically deform, with the consequences that the load cell will have to be recalibrated, or be damaged and eventually break, or suffer reduced life. The transient forces that occur when a sensor collides with the environment are hard to control and may exceed the design force limit for the sensing beam. To prevent damage from these types of events, overload protection is sometimes designed into force/torque sensors. An overload protection device must provide safe deflection of the load cell in all active directions without disturbance forces, but must provide greatly increased stiffness and strength for deflections above the safe operating point.
Thus, there is generally a need for a solution providing high tolerance to high-g forces, low noise measurements and adequate low-frequency response. More specifically, there is a need for a solution suitable for field use in seismic applications because in the marine environment, sensors are often exposed to high-g shocks.